Polyketides are a class of compounds synthesized from 2-carbon units through a series of condensations and subsequent modifications. Polyketides occur in many types of organisms, including fungi and mycelial bacteria, in particular, the actinomycetes. Polyketides are biologically active molecules with a wide variety of structures, and the class encompasses numerous compounds with diverse activities. Tetracycline, erythromycin, epothilone, FK-506, FK-520, narbomycin, picromycin, rapamycin, spinocyn, and tylosin are examples of polyketides. Given the difficulty in producing polyketide compounds by traditional chemical methodology, and the typically low production of polyketides in wild-type cells, there has been considerable interest in finding improved or alternate means to produce polyketide compounds.
This interest has resulted in the cloning, analysis, and manipulation by recombinant DNA technology of genes that encode PKS enzymes. The resulting technology allows one to manipulate a known PKS gene cluster either to produce the polyketide synthesized by that PKS at higher levels than occur in nature or in hosts that otherwise do not produce the polyketide. The technology also allows one to produce molecules that are structurally related to, but distinct from, the polyketides produced from known PKS gene clusters. See, e.g., PCT publication Nos. WO 93/13663; 95/08548; 96/40968; 97/02358; 98/27203; and 98/49315; U.S. Pat. Nos. 4,874,748; 5,063,155; 5,098,837; 5,149,639; 5,672,491; 5,712,146; 5,830,750; and 5,843,718; and Fu et al., 1994, Biochemistry 33: 9321-9326; McDaniel et al., 1993, Science 262: 1546-1550; and Rohr, 1995, Angew. Chem. Int. Ed. Engl. 34(8): 881-888, each of which is incorporated herein by reference.
Polyketides are synthesized in nature by PKS enzymes. These enzymes, which are complexes of multiple large proteins, are similar to the synthases that catalyze condensation of 2-carbon units in the biosynthesis of fatty acids. PKSs catalyze the biosynthesis of polyketides through repeated, decarboxylative Claisen condensations between acylthioester building blocks. The building blocks used to form complex polyketides are typically acylthioesters, such as acetyl, butyryl, propionyl, malonyl, hydroxymalonyl, methylmalonyl, and ethylmalonyl CoA. Other building blocks include amino acid like acylthioesters. PKS enzymes that incorporate such building blocks include an activity that functions as an amino acid ligase (an AMP ligase) or as a non-ribosomal peptide synthetase (NRPS). Two major types of PKS enzymes are known; these differ in their composition and mode of synthesis of the polyketide synthesized. These two major types of PKS enzymes are commonly referred to as Type I or “modular” and Type II “iterative” PKS enzymes.
In the Type I or modular PKS enzyme group, a set of separate catalytic active sites (each active site is termed a “domain”, and a set thereof is termed a “module”) exists for each cycle of carbon chain elongation and modification in the polyketide synthesis pathway. The typical modular PKS is composed of several large polypeptides, which can be segregated from amino to carboxy termini into a loading module, multiple extender modules, and a releasing (or thioesterase) domain. The PKS enzyme known as 6-deoxyerythronolide B synthase (DEBS) is a Type I PKS. In DEBS, there is a loading module, six extender modules, and a thioesterase (TE) domain. The loading module, six extender modules, and TE of DEBS are present on three separate proteins (designated DEBS-1, DEBS-2, and DEBS-3, with two extender modules per protein). Each of the DEBS polypeptides is encoded by a separate open reading frame (ORF) or gene; these genes are known as eryAI, eryAII, and eryAIII. See Caffrey et al., 1992, FEBS Letters 304: 205, and U.S. Pat. No. 5,824,513, each of which is incorporated herein by reference.
Generally, the loading module is responsible for binding the first building block used to synthesize the polyketide and transferring it to the first extender module. The loading module of DEBS consists of an acyltransferase (AT) domain and an acyl carrier protein (ACP) domain. Another type of loading module utilizes an inactivated ketosynthase (KS) domain and AT and ACP domains. This inactivated KS is in some instances called KSQ, where the superscript letter is the abbreviation for the amino acid, glutamine, that is present instead of the active site cysteine required for ketosynthase activity. In other PKS enzymes, including the FK-506 PKS, the loading module incorporates an unusual starter unit and is composed of a CoA ligase like activity domain. In any event, the loading module recognizes a particular acyl-CoA (usually acetyl or propionyl but sometimes butyryl or other acyl-CoA) and transfers it as a thiol ester to the ACP of the loading module.
The AT on each of the extender modules recognizes a particular extender-CoA (malonyl or alpha-substituted malonyl, i.e., methylmalonyl, ethylmalonyl, and 2-hydroxymalonyl) and transfers it to the ACP of that extender module to form a thioester. Each extender module is responsible for accepting a compound from a prior module, binding a building block, attaching the building block to the compound from the prior module, optionally performing one or more additional functions, and transferring the resulting compound to the next module.
Each extender module of a modular PKS contains a KS, AT, ACP, and zero, one, two, or three domains that modify the beta-carbon of the growing polyketide chain. A typical (non-loading) minimal Type I PKS extender module is exemplified by extender module three of DEBS, which contains a KS domain, an AT domain, and an ACP domain. These three domains are sufficient to activate a 2-carbon extender unit and attach it to the growing polyketide molecule. The next extender module, in turn, is responsible for attaching the next building block and transferring the growing compound to the next extender module until synthesis is complete.
Once the PKS is primed with acyl- and malonyl-ACPs, the acyl group of the loading module is transferred to form a thiol ester (trans-esterification) at the KS of the first extender module; at this stage, extender module one possesses an acyl-KS and a malonyl (or substituted malonyl) ACP. The acyl group derived from the loading module is then covalently attached to the alpha-carbon of the malonyl group to form a carbon-carbon bond, driven by concomitant decarboxylation, and generating a new acyl-ACP that has a backbone two carbons longer than the loading building block (elongation or extension).
The polyketide chain, growing by two carbons each extender module, is sequentially passed as covalently bound thiol esters from extender module to extender module, in an assembly line-like process. The carbon chain produced by this process alone would possess a ketone at every other carbon atom, producing a polyketone, from which the name polyketide arises. Most commonly, however, additional enzymatic activities modify the beta keto group of each two carbon unit just after it has been added to the growing polyketide chain but before it is transferred to the next module.
Thus, in addition to the minimal module containing KS, AT, and ACP domains necessary to form the carbon-carbon bond, and as noted above, other domains that modify the beta-carbonyl moiety can be present. Thus, modules may contain a ketoreductase (KR) domain that reduces the keto group to an alcohol. Modules may also contain a KR domain plus a dehydratase (DH) domain that dehydrates the alcohol to a double bond. Modules may also contain a KR domain, a DH domain, and an enoylreductase (ER) domain that converts the double bond product to a saturated single bond using the beta carbon as a methylene function. An extender module can also contain other enzymatic activities, such as, for example, a methylase or dimethylase activity.
After traversing the final extender module, the polyketide encounters a releasing domain that cleaves the polyketide from the PKS and typically cyclizes the polyketide. For example, final synthesis of 6-dEB is regulated by a TE domain located at the end of extender module six. In the synthesis of 6-dEB, the TE domain catalyzes cyclization of the macrolide ring by formation of an ester linkage. In FK-506, FK-520, rapamycin, and similar polyketides, the TE activity is replaced by a RapP (for rapamycin) or RapP like activity that makes a linkage incorporating a pipecolate acid residue. The enzymatic activity that catalyzes this incorporation for the rapamycin enzyme is known as RapP, encoded by the rapP gene. The polyketide can be modified further by tailoring enzymes; these enzymes add carbohydrate groups or methyl groups, or make other modifications, i.e., oxidation or reduction, on the polyketide core molecule. For example, 6-dEB is hydroxylated at C-6 and C-12 and glycosylated at C-3 and C-5 in the synthesis of erythromycin A.
In Type I PKS polypeptides, the order of catalytic domains is conserved. When all beta-keto processing domains are present in a module, the order of domains in that module from N-to-C-terminus is always KS, AT, DH, ER, KR, and ACP. Some or all of the beta-keto processing domains may be missing in particular modules, but the order of the domains present in a module remains the same. The order of domains within modules is believed to be important for proper folding of the PKS polypeptides into an active complex. Importantly, there is considerable flexibility in PKS enzymes, which allows for the genetic engineering of novel catalytic complexes. The engineering of these enzymes is achieved by modifying, adding, or deleting domains, or replacing them with those taken from other Type I PKS enzymes. It is also achieved by deleting, replacing, or adding entire modules with those taken from other sources. A genetically engineered PKS complex should of course have the ability to catalyze the synthesis of the product predicted from the genetic alterations made.
Alignments of the many available amino acid sequences for Type I PKS enzymes has approximately defined the boundaries of the various catalytic domains. Sequence alignments also have revealed linker regions between the catalytic domains and at the N- and C-termini of individual polypeptides. The sequences of these linker regions are less well conserved than are those for the catalytic domains, which is in part how linker regions are identified. Linker regions can be important for proper association between domains and between the individual polypeptides that comprise the PKS complex. One can thus view the linkers and domains together as creating a scaffold on which the domains and modules are positioned in the correct orientation to be active. This organization and positioning, if retained, permits PKS domains of different or identical substrate specificities to be substituted (usually at the DNA level) between PKS enzymes by various available methodologies. In selecting the boundaries of, for example, an AT replacement, one can thus make the replacement so as to retain the linkers of the recipient PKS or to replace them with the linkers of the donor PKS AT domain, or, preferably, make both constructs to ensure that the correct linker regions between the KS and AT domains have been included in at least one of the engineered enzymes. Thus, there is considerable flexibility in the design of new PKS enzymes with the result that known polyketides can be produced more effectively, and novel polyketides useful as pharmaceuticals or for other purposes can be made.
By appropriate application of recombinant DNA technology, a wide variety of polyketides can be prepared in a variety of different host cells provided one has access to nucleic acid compounds that encode PKS proteins and polyketide modification enzymes. The present invention helps meet the need for such nucleic acid compounds by providing recombinant vectors that encode the FK-520 PKS enzyme and various FK-520 modification enzymes. Moreover, while the FK-506 and FK-520 polyketides have many useful activities, there remains a need for compounds with similar useful activities but with better pharmacokinetic profile and metabolism and fewer side-effects. The present invention helps meet the need for such compounds as well.